1. Field of the Invention
This invention relates to hydraulic shock absorbers mounted between the sprung portion and unsprung portion of a vehicle, and more particularly to a control system for adjustable-damping force hydraulic shock absorbers.
2. Description of the Prior Art
Conventionally, adjustable-damping-force hydraulic shock absorbers mounted between the sprung portion and unsprung portion of a vehicle have been proposed. The shock absorbers each include an adjusting element rotated by an electric motor to a desired angular position in accordance with the conditions under which the vehicle runs, thereby adjusting the damping force which the shock absorbers produce satisfactorily to improve driving comfort and stability. A switch selects the desired one of three "hard", "normal" (medium) and "soft" damping force setting positions. When the desired one of the three setting positions is selected, a selection reference signal generator accordingly produces a corresponding selection reference signal which is then compared by a comparator to a signal indicative of the angular position of an output shaft of the motor and hence the angular position of the adjusting element. If the reference signal is not equal to the angle-indicative signal, a signal indicative of this will be produced by the comparator, which causes the motor to rotate until the reference signal and angle-indicative signal match. This controls the rotation of the adjusting member within a cylinder, thereby adjusting the amount of hydraulic fluid which bypasses a damping passage between two hydraulic chambers, and hence the resistance of the fluid within the cylinder to displacement between the sprung and unsprung masses.
Occasionally, problems occur in the control device for a particular shock absorber; for example, the motor may seize, the harness for the motor may break, or the detector which senses the angular position of the motor may malfunction. In such cases, the shock absorbers which are controlled by the faulty control device and by the remaining, working control devices may differ in regard to the adjustable damping force. For example, even if three of the shock absorbers associated with working control devices may be set to the desired damping-force setting, the other shock absorber associated with faulty control device may remain in an undesired state. In that case, the unequal damping forces among the shock absorbers will degrade the driving stability of the vehicle if the differences among the damping forces are great.
In the above shock absorbers, on the other hand, the motor is driven to rotate the adjusting member such that an opening in the adjusting member is aligned with a desired one of a plurality of orifices in the stud, thereby establishing a desired damping force, as briefly mentioned above. For that purpose, the motor must be stopped accurately at the desired angular position. However, the motor and the adjusting member driven by the motor have noticeable moments of inertia, so that after the supply of driving current to the motor has stopped, the motor and the adjusting element will rotate unnecessarily due to their inertia. As a result, the desired orifice in the stud and the opening in the adjusting member may not exactly align, so that the desired damping force may not be achieved.
Furthermore, if the motor which drives the adjusting element is frozen, or the output terminals of the motor driving unit are short-circuited for some reason, excess current will flow from the power supply to the motor, so that the harness between the motor and the motor driving unit may overheat and burn, or that the motor driving unit itself will overload and hence be damaged.
A detector which senses the angular position of the motor is provided near, or integral with, the associated shock absorber for that purpose. In addition, the rest of the control device including the comparator, the motor driving unit, the selection switch, etc., is disposed in the vehicle compartment or behind the instrument panel as is the case with the controllers for other electrical equipment installed in the vehicle. In other words, the detector and the rest of the control system are separated from each other. Since the source of electric power will be provided with the rest of the control system, electric power must be supplied to the detector via electrical wiring. If a short-circuit occurs in the wiring, the wiring and other harness may be burnt. Furthermore, if the detector cannot perform its sensing function because of problems in the power supply system, the damping-force control device may continuously drive the motor.
The power which drives the loads such as the motor is usually derived from the battery or a generator mounted on the vehicle. However, the voltage of such sources of power is very likely to fluctuate, so that the driving force for the motor will not be constant. This is likely to occur during operation of the vehicle and is also likely to be influenced by high-voltage surges peculiar to the vehicle, which constitute a defect in terms of control. As an example, this may result in fluctuations in reference signals supplied from a selection reference signal generator to a comparator, thereby causing the control system to operate in an unstable manner. Furthermore, operation of other elements of the electrical system, such as switching off the starter motor, may produce a counterelectromotive force of, for example, hundreds of volts which may greatly damage the electrical elements of the control system.